Full range electroacoustic transducers



July 14, 1964 s. R. RICH 3,141,071

FULL RANGE ELEcTRoAcousTIc TRANsDucERs ATTORNEY July 14, 1964 s. R. RICH 3,141,071

FULL RANGE ELEcTRoAcoUsTIc TRANsDUcERs Filed July 18, 1960 2 Sheets-Sheet 2 42 46 z3/42 2/,2 u A: l

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STANLEY R. RICH BWM@ ATTORNFY transducing process.

United States Patent 3,141,071 FULL RANGE ELECTRACOUSTIC TRANSDUCERS Stanley R. Rich, Hartford, Conn., assigner, by mesne asssignments, to Alfred H. Rosen, Newton, Mass. Filed July 18, 1960, Ser. No. 43,435 40 Claims. (Cl. 179-1155) This invention relates in general to full-range audio electroacoustic transducers, and more especially to novel electrodynamic transducers of the active diaphragm type.

By the active diaphragm types of transducer, I mean that type in which the radiating diaphragm itself is driven substantially over its entire radiating area by an element or elements of the electrical circuit of which the transducer is a part. Examples of this type of transducer are the electrostatic microphone and loudspeaker, wherein the diaphragm is one plate of a capacitor, and the ribbon microphone, in which the diaphragm carries currents corresponding to sound pressure variations incident upon the diaphragm. In the art of faithful conversion of auditory sound energy into electrical energy, and vice versa, transducers of the active diaphragm type have long been recognized as superior to those of the passive diaphragm type, which interpose an inert element in the Passive diaphragms in loudspeakers, such as conventional cone-shaped elements driven by voice coils, produce transient distortion by virtue of energy storage. Elastic Wave energy remains in such passive elements after the cessation of a given driving electrical impulse, and sound continues to be radiated until the stored energy dies away. Thus, in his book Acoustics (Prentiss Hall, 1957) at page 215 Professor Joseph L. Hunter comments, concerning the ribbon microphone, In many respects, this microphone, consisting of a single ribbon, and having very desirable frequency response and directivity is a triumph of applied acoustics. Electrostatic loudspeakers can be described in even more glowing terms, yet speakers of the passive diaphragm type, particularly the well-known cone-type speakers driven by a voice coil, have pre-empted all but a small segment of the speaker market, and that segment is occupied only by costly and complicated transducers, exclusively electrostatic until the advent of the present invention.

Attempts to provide loudspeakers of the ribbon type have not met with success. A ribbon is not naturally aperiodic, and one approach, exemplified by Gerlach Patent 1,557,356, has been to provide the ribbon with transverse corrugations and hold it under tension. I have built transducers according to the teaching of Gerlach, and found that ribbons, made of aluminum and corrugated and supported as taught in this patent, have many resonances, which I have found to degrade the reproduction by imparting a tinny sound, especially at output levels comparable with actual spoken voice or musical instrument sounds. Even when used in a microphone, where motional excursions are smaller than in a loudspeaker, ribbons have continued to exhibit resonances, as appears from Eckardt Patent 2,608,265 in which the ribbon is corrugated at one end and mass-loaded at the other end, to suppress spurious Vibrations, especially in the third and other odd harmonic modes. Further, ribbon transducers have extremely low electrical impedance and cannot be coupled to existing amplifiers without special impedance matching networks or transformers.

While electrostatic transducers have been built with considerably greater success insofar as the problem of diaphragm resonance is concerned, these transducers suffer from the very obvious and insurmountable requirement for a high-voltage supply, and a host of difficulties which accompany its use. Since no modern electronic en- ICC tertainment equipment, except a television receiver, inherently employs a sufliciently high voltage supply, such a supply must be furnished in most cases solely for the use of the loudspeaker, thereby adding to its cost, and severely limiting the range of applicability of, and hence the market for, such speakers. Furthermore, the forces available per unit diaphragm area in electrostatic transducers are so small that extremely large diaphragms are necessary in order to reproduce the entire audio range at useful volume level. While electrostatic high frequency transducers (tweeters) can be made reasonably small, full range electrostatic speakers usually have diaphragms which are several square feet, for example, six or more square feet, in area, six square feet being approximately the minimum size full range electrostatic speaker which is presently commercially available.

It is the principal object of the present invention to provide an improved electroacoustic transducer of the active diaphragm type. Another important object of the invention is to provide such a transducer which does ,not require any special voltage or power supply. Still another important object of the invention is to provide such a transducer which is devoid of resonances at least to the same extent as the best quality full range electrostatic speakers currently available. Another object of the invention is to provide such a transducer in the form of a unit of convenient size which can be used singly or in multiple, without introducing complex circuit requirements for linking such units for multiple operation. Additional objects are to provide such units which can be fabricated easily of readily available materials, are of a design which introduces no critical assembly or utility problems, can be used with existing amplier equipment by direct connection and without requiring any special impedance matching networks or transformers, and in combination with other speakers if desired, and which are of low cost as compared with even modest quality speakers of the passive diaphragm (e.g., paper cone) type.

According to the invention, these objects are generally achieved with a novel electrodynamic transducer comprising means to establish a sheet-like magnetic iield which is substantially uniform within prescribed limits in the thickness direction at any part thereof, a continuous substantially imperforate diaphragm disposed within said limits and extending throughout said iield normal to the thickness dimension, and a continuous electrical conductor mounted on said diaphragm and extending everywhere substantially normal to the lines of force of said tield. More specifically, the magnetic iield is established between first pole means establishing a pole of one polarity and second pole means surrounding and spaced from the iirst and establishing a pole of the other polarity, and the diaphragm spans the entire area surrounded by the second pole means. Preferably, the lirst pole means is of an elongated rectangular shape, and the second pole means is a rectangular annulus the inner periphery of which is nearly everywhere equidistant from the first pole means, in order to maximize the uniformity of the iield, and the continuous conductor is in the form of a iiat rectangular spiral. In a preferred embodiment, the iirst pole means comprises two iiat, elongated rectangular permanent magnets, each polarized on two opposite long flat faces, supported with like poles confronting and spaced apart a distance which establishes said prescribed limits. In such an embodiment the second pole means can be nonpermanent magnet magnetizable material, disposed at the periphery of the field, and in such a case the thickness of said second pole means is substantially equal to said distance. In a practical realization of this embodiment, said second pole means comprises two rectangular magnetically permeable soft iron rings, each of a thickness equal to approximately half said distance, and soft iron yoke means supports each of said magnets from one of said rings; in this realization, said diaphragm is conveniently clamped peripherally between said two rings to complete the assembly thereof. In all embodiments the diaphragm is substantially imperforate and is hermetically sealed across the field.

Other and further advantages, objects and features of the invention, more especially of such practical realizations of the invention, will become apparent from the following description of certain embodiments thereof. This description refers to the accompanying drawings, wherein:

FIG. 1 is a top View of a first embodiment of the invention;

FIG. 2 is a cross section on line 2 2 of FIG. 1;

FIG. 3 is a partial View similar to FIG. 2 illustrating the magnetic field achieved with this embodiment;

FIG. 4 is an enlarged partial View of an active diaphragm according to the invention;

FIG. 5 is a top View of another embodiment of the invention;

FIG. 6 is a longitudinal section on line 6 6 of FIG. 5;

FIG. 7 is a sketch, in half-scale, useful to explain FIGS. 1 and 5;

FIG. 8 is a comparative sketch of another embodiment of the invention;

FIG. 9 is a partial top view of another embodiment of the invention, similar to FIGS. 1 and 5 but employing a different diaphragm structure;

FIG. 10 is a cross section on line 10-10 of FIG. 9;

FIG. l1 is an enlarged cross section showing details of a diaphragm suitable for use in the embodiment of FIG. 9;

FIG. 12 is a plan view of a modification of the diaphragm structure of FIG. 1l;

FIG. 13 is a section along line 13-13 of FIG. 12;

FIG. 14 is an enlarged partial cross section of another active diaphragm according to the invention; and

FIG. 15 illustrates one manner of baffle mounting transducers of the invention.

Referring now to FIGS. 1 and 2, a first elongated rectangular permanent magnet 11, polarized to have a north pole on one long fiat face 11A and a south pole on the opposite long flat face 11B, is supported from a first rectangular annular pole piece 21, by means of two standoff members 11.1 and 11.2 each of which is mounted approximately midway along and normal to one of the long sides 21.1 and 21.2, respectively, of the pole piece 21. A cross bar 11.3 connects the two free ends of the stand-off members 11.1 and 11.2, and a spacer bar 11.4 is connected at its midpoint to the midpoint of the cross bar 11.3 running perpendicular to its long dimension. The magnet 11 is connected, as by a suitable cement, at its south pole face 11B to one surface of the spacer 11.4. The pole piece 21, stand-off members 11.1 and 11.2, cross bar 11.3 and spacer bar 11.4 are all made of a magnetizable nonpermanent magnet material such as soft iron. As shown in FIG. 1, these members may be assembled by means of screws 25 or the like. They may also be cast or otherwise formed as a single piece having no joints therein. A similar magnet assembly consisting of a second permanent magnet 12 polarized in like fashion to the first permanent magnet 11, a soft iron annular rectangular shaped pole piece 22 and stand-off elements 12.1 and 12.2 mounted approximately at the midpoints of and normal to the long sides 22.1 and 22.2, respectively, thereof, connected at their free ends by a cross bar 12.3 and having a spacer bar 12.4, is provided, with the south pole of the permanent magnet 12 confronting the spacer bar 12.4. The annular pole pieces y21 and 22 have substantially equal thicknesses and the magnet assemblies are assembled into a single transducer @naar unit by means of bolts 26, for example, passing through and holding the annular pole pieces 21 and 22 confronting each other so that the north poles of the permanent magnets 11 and 12 confront each other and are spaced apart a distance approximately the same as the combined thickness of the two pole pieces 21 and 22.

A fiat substantially imperforate diaphragm 31, one suitable structure for which is shown in FIG. 4 which shows a woven, for example, nylon fabric 31.1, and bearing thereon a flat continuous spiral of wire 32, preferably of a material such as aluminum, embedded in an elastomer 33, is held between the annular pole pieces 21 and 22 completely sealing the opening Within these pole pieces, with the articulated wires 32 disposed approximately midway between the confronting north poles of the two permanent magnets 11 and 12. As is seen more clearly in FIGS. 1 and 2, the articulated conductors of the continuous fiat spiral of wire 32 substantially completely fill the rectangular opening within the rectangular annular pole pieces 21 and 22. Terminals 32.1 and 32.2 are provided at the ends of the wire 32.

Referring now to FIG. 3, the two permanent magnet north poles confronting each other are spaced apart a distance, as is mentioned above, which is approximately equal to the combined thicknesses of the two annular pole pieces 21 and 22, and due to the continuous soft iron path from the south pole of each magnet to the inner surfaces of the pole pieces, the inner surfaces of these pole pieces are south poles. The result of this conguration is to provide a substantially fiat sheet of magnetic flux 15, having its shortest lines of magnetic force essentially straight lines as illustrated at 15.1. This field is substantially uniform in thickness at any point in it; that is, at any point in the field there is substantially no change in field density within prescribed limits measured normally from such point. The prescribed limits here referred to are substantially the distance between the confronting permanent magnet pole faces of like polarity, or the combined thickness of the two annular pole pieces 21 and 22. Any fringing field lines which tend to curve, as shown at 15.2 have longer paths to traverse and are, therefore, held to a minimum, and are essentially outside these prescribed limits.

The confronting permanent magnet north poles (here north poles but it will be appreciated that they might be chosen to be the south poles) provide an Opposed-pole field which achieves a maximum density of magnetic flux in this flat sheet-like field. As is illustrated in FIG. 1, the shortest lines of force of this field are substantially all directed in parallel groups, and extend equal distances from the north pole to the south pole over substantially the entire annular path between the permanent magnets and the annular pole pieces, as represented by one permanent magnet 11 and one annular pole piece 21 in FIG. 7. Only at the corners will there be any longer magnetic field paths, and these are held to an absolute minimum by virtue of the choice of a rectangular permanent magnet pole and rectangular annular pole piece structure.

By virute also of the opposed-pair magnet and yoke structure described above and shown most clearly in FIG. 2, consisting of two like opposed structural subassemblies, holding the diaphragm 31 midway between them, the articulated conductors of the continuous fiat spiral of wire 32 are all maintained substantially in the center, as referred to the thickness dimension, that is, substantially midway between the above-mentioned prescribed limits, of the ffux sheet 15. Thus, any elemental conductor of the wire 32 at any point in the flux sheet can execute substantially large excursions in the direction normal to the flux sheet without encountering any noticeable change in magnetic field density as a result of such movement. Accordingly, when the configuration ilustrated in FIGS. 1 and 2 is employed as a loudspeaker the diaphragm 31 moves substantially isophasally, and there is virtually no intermodulation distortion. Even for low frequencies which cause relatively large excursions of the diaphragm 31, each elemental conductor of the articulated spiral conductor 32 remains in a substantially constant and uniform magnetic field, so that high frequencies are not distorted, and no intermodulation distortion can result.

Substantial uniformity of the flux sheet is assured by the provision of the two permanent magnets 11 and 12. In permanent magnets the lines of force leave the magnetic domains substantially perpendicular to the pole surfaces, and to enhance this effect, I have provided substantially flat elongated rectangular pole surfaces (as at 11A) at each pole. By bringing a pair of like poles into opposition, the magnetic field is forced by repulsion into a flat flux sheet. The soft iron return loops providing together a south pole face which is substantially the same thickness as the flux sheet as determined by the confronting permanent magnet north poles, achieved by providing the annular pole pieces 21 and 22 of suitable thickness for this purpose, help to maintain the flux sheet 15 flat and uniform, as is described above. It is important to bear in mind that the opposed permanent magnets have no soft iron or nonpermanent pole faces opposing one another, or in the working gap between them, and this is an important element in assuring the provision of a uniform magnetic field from the center of the gap. This feature of the invention is important for assuring that flux density is uniform in the axial direction, that is, in any line perpendicular to the diaphragm 31. With this structure I am able to provide a gap on the order of one-quarter inch between the confronting permanent magnet pole pieces, Iand a field strength throughout the flux sheet 15 of approximately 2000 Gauss, through which the diaphragm can execute motional excursions.

The fact that I cause the magnetic flux to issue uniformly from the confronting poles of the permanent magnets permits me to use an articulated (continuous spiral) winding 32 bonded to ya relatively light diaphragm 31 in which each elemental part of the winding is acoustically active. It is important also to note that the major portion of the flux sheet 15 is in a region which is not blocked to a listener by the pole pieces 11 and 12 or 21 and 22, thereby assuring that the active diaphragm 31 is substantially not blocked acoustically by the pole pieces so that the major portion of the active diaphragm 31 is free to radiate sound directly tot the listener. This is especially important for faithful reproduction of the high frequencies. On the other hand, I provide that the articulated winding 32 extends to the region between the confronting pole faces of the pole pieces 11 and 12 so that, especially for low frequencies, the entire diaphragm is driven isophasally, thereby preventing any out-of-phase motion of the diaphragm at low frequencies. This is in sharp contrast with prior art structures represented by the disclosure of Gueritot Patent 1,523,262 in which opposed opposite poles (i.e., one north pole and one south pole) having conical faces, one convex and the other concave, and having a conical spiral conductor between them, block almost entirely any path from the vibrating conductor to a listener. Further, in the Gueritot device, acoustic resonances are inherent in his Various confined passages and orifices, resulting in degradation of sound quality.

In the embodiment-of an active diaphragm shown in FIG. 4, I prefer to employ aluminum wire about 0.004 to 0.020 inch in diameter, with enamel (for example, Formvar) insulation, held on the fabric diaphragm 31.1 by means of a suitable cement 33 such as soft rubber cement. It will be understood that I may employ other diaphragm materials, such as thin sheets of suitable plastic (e.g., Mylar), or of a metal such as aluminum if desired (as hereinafter described) and that the articulated winding 32 need not be embedded in the cement 33 but can protrude from the cement, if desired. Also I may manufacture the articulated conductor 31 by printed circuit techniques, if desired, and it is therefore to be understood that the diaphragm structure which I have shown in 6 FIG. 4 is by way of example only. Otherl embodiments of suitable active diaphragms are hereinafter described in connection with FIGS. 9 to 14, inclusive.

FIGS. 5 and 6 illustrate an alternative arrangement of the magnetic yoke structure in which the soft iron pole rings 21 and 22 have stand-off members 11.5 and 11.6 normally mounted thereon at the mid regions of the short sides 21.3 and 21.4, respectively, of the first ring 21, and 12.5 and 12.6 similarly mounted on the short sides 22.3 and 22.4, respectively of the second ring 22, each pair of stand offs being spanned a-t the free ends by a soft iron bar 11.7 and 12.7, respectively. The spacer bars 11.4 and 12.4 are attached to the cross bars 11.7 and 12.7, respectively as is shown more clearly in FIG. 6. The structure of FIGS. 5 and 6 differs from the structure of FIGS. 1 and 2 only in this alternative arrangement of the cross bars. Either structure is equally simple to construct.

FIG. 8 illustrates a circular arrangement in which a permanent magnet 11.9, shown here as having a north pole confronting the flux sheet, is provided with an annular pole piece 21.9 of circular form to provide a south pole. Here the field lines shown by the arrows are all radial so that they are not parallel to each other. While a circular structure as shown in FIG. 8, and embodying the other features of my invention, is operative, I prefer the rectangular structure of FIG. 7 for the reasons mentioned above. I contemplate also that other geometries may be used, including, without limiting the generality thereof, but as examples, ellipses, ovals, squares, etc.

FIGS. 9 and 10 illustrate -another embodiment of the invention employing a diaphragm 41 which is peripherally attached to a motion compliance rim 42, which in turn is peripherally held between the annular pole pieces 21 and 22. For the purpose of simplifying the illustration of the features of FIGS. 9 and 10 which are different from the embodiments of FIGS. 1 and 5, parts which are alike in all of these embodiments have been omitted from FIGS. 9 and 10. These parts are the permanent magnets 11 and 12 and the parts 11.1 to 11.4 and 12.1 to 12.4, inclusive, for supporting them from the annular pole pieces, and the articulated spiral winding 32. It will be understood that such parts are included in a complete embodiment according to FIGS. 9 and l0.

The diaphragm 41 may be constructed as is shown in FIG. ll. Here a sheet of dielectric material such as a polyester film characterized by high tensile strength available commercially under the trademark Mylar), of the order of 0.5 mil. inch thick, provides a supporting membrane 46 for the conductors 32 of an articulated spiral similar to the spiral conductor shown in FIG. l. The thickness of the supporting membrane may be in the range approximately 0.2 to 2.0 mil. inch. The conductors are held on the supporting membrane 42 by a soft cement 43 of a type which as in FIG. 4 is permanently nonrigid, such as plasticized flexible rubber cement. While the polyester film ymaterial can, due to its high tensile strength, be regarded as stiff, compared to the woven fabric 31.1 of FIG. 4, the supporting membrane 46 made of it is quite flexible at this thickness. Such a polyester film material is, however, hard compared to a woven fabric. In order to absorb and dissipate flexural energy in the supporting membrane 46 and to attribute a piston-like action to the diaphragm 41, I have intimately bonded to the membrane substantially over the entire surface opposite to the surface bearing the conductors 32, an absorptive material 44, which may be a woven nylon mesh similar to the fabric 31.1 of FIG. 4. Alternatively, the absorptive material 44 may be a polyfibrous paper or a plasticized film material. The intimate bond for a fabric or a paper absorptive material is achieved preferably with a permanently nonrigid cement 45, such as a plasticized flexible rubber. Substantially any material and bond which together will cause energy loss when flexed, and will, therefore, dissipate flexural vibrational energy arising in the hard material of the supporting membrane 46, is suitable for use as the absorptive material. Preferably, the absorptive material achieves greater than critical damping of the diaphragm 41 with respect to flexural vibration.

A diaphragm 41 constructed as described above with reference to FIG. 11 will execute substantially piston-like action at audio frequencies. The compliance rim 42 provides a mounting for the diaphragm 41 which does not inhibit freedom of the diaphragm to execute piston-like motion at the edges. To this end, the compliance rim may be transversely arched, as is shown in FIG. 10. It is preferably made of a highly flexible sheet material, such as rubber, preferably not thicker than the support-- ing membrane 46.

FIGS. 12 and 13 illustrate modifications of the diaphragm 41 of FIGS. 10 and l1 for the purpose of enhancing its audio frequency response. In FIGS. 12 and 13, the articulated spiral 32 is only partially shown, in order to simplify the illustration. A mass loading element 51, elongated and running completely around one surface of the supporting membrane 46 is attached to the membrane, as by a suitable cement (not shown). The mass loading element may be made, for example, of copper wire one-sixteenth inch in diameter. With such a mass loading member, I have reduced from approximately 120 cycles per second to approximately 40 cycles per second the natural resonance frequency of a diaphragm 41 made according to FIG. 11, and about two inches wide and 4 inches long within the compliance rim 42. At the same time, I further enhanced the piston-like action of the diaphragm at low frequencies.

In order further to insure true piston-like action of the diaphragm 41 at low frequencies, I may add transverse stilleners 52. These may be made, for example, of copper Wire segments, having a natural frequency of flexural self-resonance in the vicinity of 500 cycles per second or higher. Like the mass loading element, they are cemented to the supporting membrane 46, passing directly over the conductors of the driving articulated spiral 32, if desired. Both the peripheral mass loading element 51 and the transverse stiffeners 52 will be electrically insulated from the driving spiral 32 if made of metal. While the stiffcners 52 further insure piston-like action of the diaphragm 41 at low frequencies, where motional excursions are of higher amplitude, they are damped by the flexural energy absorbing characteristics of the absorptive material 44 and become substantially inert at high frequencies.

I have produced substantially uniform sound output from 40 to 15,000 cycles per second, with freedom from distortion which is comparable to that of the highest quality wide-range electrostatic transducers currently available, and superior to that of any available transducers of the passive diaphragm type, using a speaker constructed according to FIGS. to 13, inclusive, with a diaphragm 2 x 4 inches in area within the compliance rim, the speaker being mounted in an infinite baffle as is shown in FIG. 15.

FIG. 14 is an enlarged cross-sectional view of another active diaphragm of the invention which achieves a substantially piston-like motional displacement in the flux sheet 15, by virtue of phenomena not present in the diaphragms heretofore described. In this figure, the supporting membrane 56, for the conductors 32, is made of aluminum sheet about 0.5 mil. inch thick, and the conductors 32 are cemented to one surface of the aluminum sheet as by a suitable, preferably permanently flexible cement 57. It desired, a diaphragm 41 may be constructed, using the electrically conductive membrane 56 in place of the dielectric membrane 46, but otherwise identical to and having one or more of the features of FIGS. 9 to 13, inclusive, and such a diaphragm will be operative as fully IOS .and as satisfactorily as the diaphragms employing the dielectric supporting membrane. I have discovered, however, that equally satisfactory results can be achieved with a diaphragm constructed as shown in FIG. 14, and 'used with a compliance rim. While I do not intend to be limited by any explanation of these results, I believe an explanation of the action of a diaphragm according to FIG. 14 in the flux sheet 15 is probably as follows.

Eddy currents are induced by the driving spiral 32 in A.the electrically conductive membrane 56 when current is Vpassed through the spiral conductors. With these eddy vcurrents present, the membrane behaves as a single-turn secondary winding relative to the driving spiral. The jprimary and secondary windings are, however, fastened together by the cement 57, and both repose in the same magnetic field, namely the flux sheet 15. As in any transformer, there is a phase difference between the primary and secondary currents, which difference is in the vicinity of because both primary and secondary cirt-cuits are predominantly resistive. The resultant motion 4of the entire structure in the flux sheet 15 faithfully reproduces the entire range of audible frequencies. I believe further that the motion of the electrically conductive ,membrane 56 in the flux sheet 15 induces voltages in the .membrane which react with the magnetic field in a sense which tends to stop the motion of the membrane, thereby contributing a form of dynamic braking to the diaphragm lwhich improves its transient response. At the end of a Icurrent pulse in the driving spiral 32 this dynamic braking brings the membrane 56 to a substantially instantaneous halt, so that sound is not radiated after the current vpulse has passed.

In summary, when current is present in the driving spiral 32, both the spiral conductors and the membrane 56 drive the diaphragm into corresponding motional excursions, and when the current ceases the membrane acts 4as a dynamic brake.

It is thus seen that at least two ldifferent types of active fdiaphragms can be used in the present invention. These are:

(a) Diaphragms employing a supporting membrane which is inherently lossy with respect to flexural vibrations, and will not support such vibrations; and

(b) Diaphragms made of hard materials which inherently can support flexural vibrations, with measures taken to overcome any disadvantage flowing from this property.

Of the former type, a woven fabric as shown in FIG. 4 can be used; alternatively, paper, rubber or plasticized sheets, or other membrane or film-like materials having this property can also be used. Of the latter type, electrically nonconductive, or dielectric materials can be used with one or more of the mechanical modifications illustrated in FIGS. 11, 12 and 13, or electrically conductive materials can be used with or without such modifications, as desired.

In FIG. 15, a speaker, generally represented at 61, is mounted on a baille 63 through an opening 64 therethrough. Mounting is achieved by attaching the outer portions of the annular pole pieces 62 to the baflle 63. Reference character 62 in FIG. 15 represents the two annular pole pieces 21 and 22 of FIG. 1, for example, assembled as is shown in FIG. 2. FIG. 15 illustrates one manner of fitting a speaker according to the invention with an infinite baille. It is by way of illustration only, and not intended to be limiting on the invention. This figure illustrates that, with the diaphragm 65 sealed completely across the opening in the annular pole pieces, innite acoustic baffling of the active diaphragm of the present invention is easily achieved.

Reviewing the advantages of my invention they are as follows:

(l) My magnet configuration has no nonpermanent magnetic pole pieces opposing each other or in the working gap; this prevents or substantially reduces magnetic field fringing and assures a magnetic field which is essentially a flat sheet as is shown in FIG. 3, and which is uniform in the direction perpendicular to the two working pole faces; I call this a iux sheet which is uniform in the thickness direction; the soft iron return poles (S in FIG. 3) are essentially the same thickness as the flux sheet to preserve this condition; a suitable thickness for the flux sheet which I have achieved in practice is approximately one-quarter inch;

(2) With this flux sheet there is no intermodulation distortion with the diaphragm 31 moving at all frequencies in the audible range; this is due to the fact that the flux sheet is essentially uniform across the gap so that large excursions of the diaphragm at low frequencies do not move the fiat spiral winding 32 into a field of different intensity;

(3) The fiux sheet issues uniformly from center line of the opposed permanent magnetic poles in the lateral direction, enabling me to use a single articulated winding 32 of practically any desired length and thereby to obtain increased impedance of practically any desired value (as for example, any value from 1 to 16 ohms) across the terminals 32.1 and 32.2; this enables me to operate without using special transformers to match a low impedance; for example, I can operate my speaker from the output of a transistor amplifier without an output coupling transformer; it also enables me to achieve a flat spiral motor winding 32 for my active diaphragm 31 in which the entire winding from the center to the periphery is acoustically active and has no dead spots,k so that there are no dead spots in the diaphragm 31, with the result that the entire diaphragm moves in the same phase, that is, isophasally for all frequencies in the audible range;

(4) The articulated winding 32 is of light weight and is bonded to a light-weight diaphragm 31; the diaphragm is held at its periphery between the rectangular annular pole pieces 21 and 22, which thus serve a dual purpose, namely, to mount the diaphragm, and to function as pole pieces in the magnetic flux circuit;

(5) The diaphragm hermetically seals the entire opening within the pole pieces 21 and 22, or in other words the diaphragm extends throughout the entire iiux sheet 15, so that there is no opportunity for vibration from the one face of the diaphragm to appear at the opposite face and cancel vibration from the diaphragm at any frequency;

(6) The major portion of the magnetic eld and hence of the active diaphragm 31, is not blocked acoustically by the pole pieces 11 and 12, so that the major portion of the active diaphragm is free to radiate sound directly to the listener;

(7) The provision of a magnetic field structure consisting of two opposed pole structures achieves cancellation of even order harmonics due to the symmetry of the magnetic field; and

(8) The elongated magnet structure, as described in connection with FIG. 7 assists in achieving, a uniform magnetic field thereby further aiding the reduction of intermodulation distortion.

The embodiments of the invention which have been described and illustrated herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also within the spirit and scope of the invention, and it is intended that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

What is claimed is:

1. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed area, said ux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, a substantially imperforate membrane coated on one side with an elastomer disposed in and extending throughout said first-named area, and a continuous flat spiral of elongated electric conductor means embedded in said elastomer and distributed over the major portion of said membrane substantially from said periphery to and including at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said membrane substantially throughout said firstnamed area.

2, Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed area, said flux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said eld extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to said diaphragm means and distributed over Vthe major portion thereof substantially from said periphery to and including at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area, said diaphragm means comprising a supporting membrane having on one side adhering thereto a layer of a relatively softer material substantially to dissipate vibrational energy due to iiexure of said membrane, and said conductor means being attached to the other side of said membrane.

3. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed area, said flux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said rst-named area, and a continuous flat spiral of elongated electric conductor means attached to said diaphragm means and distributed over the'major portion thereof substantially from said periphery to and including at least a portion of said midregion, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area, said diaphragm means comprising a supporting membrane for said conductor means, said membrane having the same shape as but being smaller than said rst-named area, and motion compliance means made of sheet-form substantially imperforate flexible material peripherally supporting said diaphragm means in said first-named area.

4. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of il i.

magnetic fiux extending over a prescribed area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending generally radially in direction between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named arca being a minor fraction of said first-named area, a substantially imperforate membrane disposed in and extending throughout said first-named area, mass loading means attached to said membrane, and a continuous fiat spiral of elongated electric conductor means attached to said membrane and distributed over the major portion thereof substantially from said periphery to and inciuding at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said membrane substantially throughout said first-named area.

5. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area, said flux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said rst and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof substantially from said periphery to and including at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area, said diaphragm means comprising a supporting membrane for said conductor means, and mass loading means in the form of an elongated mass member peripherally attached to and extending around substantially the entire periphery of said membrane.

6. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area. substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof substantially from said periphery to and including at least a portion of said midregion, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area, said diaphragm means comprising a supporting membrane for said conductor means, mass loading means in the form of an elongated mass member peripherally attached to and extending around substantially the entire periphery of said membrane, and elongated members exhibiting stiffness at frequencies above approximately 500 cycles per second attached to said membrane spanning the same.

7. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, a substantially imperforate membrane of woven fabric coated with an elastomer disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to said membrane and distributed over the major portion thereof substantially from said periphery to and including at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said membrane substantially throughout said first-named area.

8. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, a substantially imperforate electrically conductive sheet diaphragm disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to and electrically insulated from said diaphragm and distributed over the major portion thereof substantially from said periphery to and including at least a portion of said mid-region, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area.

9. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending between said first and second pole means in like sense from the periphery to said mid-region, the area of said rst pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and a continuous fiat spiral of elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof substantially from said periphery to and including at least a portion of said midregion, whereby an alternating current in said conductor means will produce substantially isophasal corresponding alternating motion of said diaphragm substantially throughout said first-named area, said diaphragm means comprising a sheet of dielectric material having thickness in the range approximately 0.2 to 2.0 mil. inches and said conductor means being aluminum wire in the range approximately 0.4 to 2.0 mil. inches in diameter.

l0. For use in an electrodynamc transducer, a cornbined magnetic pole and diaphragm support structure, comprising a fiat rectangular annulus, an elongated permanent magnet having poles on two opposite longer fiat faces thereof, a bridge at one fiat side of said annulus joining the intermediate regions of two parallel arms of said annulus, said bridge having two legs standing up one from each of said intermediate regions, and a center portion connected to said legs at the free end of each, said magnet being connected at one of said longer fiat 13 faces to the underside of the intermediate portion of said bridge, said bridge being dimensioned to provide that the remaining one of said longer fiat faces lies in a plane which is substantially parallel to and at least in the close vicinity of one of the fiat sides of said annulus, said bridge being made of magnetizable material.

l1. For use in an electrodynamic transducer, a combined magnetic pole and diaphragm support structure, comprising a fiat rectangular annulus, an elongated pole piece having a flat elongated face on one side thereof, a bridge at one of the fiat sides of said annulus, joining the intermediate regions of two opposite parallel arms of said annulus, said bridge having two legs standing up one from each of said intermediate regions and a center portion connected to said legs at the free end of each, said pole piece being connected to the underside of the intermediate portion of said bridge, said bridge being dimensioned to provide that said flat elongated face lies in a plane which is substantially parallel to and at least in the close vicinity of one of the flat sides of said annulus, said fiat elongated face extending across said annulus substantially parallel to one pair of opposite parallel arms thereof, said pole piece and said bridge being made of magnetizable material.

12. For use in an electrodynamic transducer, a fiat elongated rectangular annulus, an elongated pole piece having a fiat elongated face on one side thereof, a bridge at one of the fiat sides of said annulus joining the intermediate regions of two opposite parallel arms of said annulus, said bridge having two legs standing up one from each of said intermediate regions and a center portion connected to said legs at the free end of each,

Vsaid pole piece being connected to the underside of said bridge, said bridge being dimensioned to provide that said flat elongated face lies in a plane which is substantially parallel Vto and at least in the close vicinity of the fiat sides of said annulus, said flat elongated face having its long dimension parallel to the longer arms of said annulus, `said pole piece and said bridge being made of magnetizable material.

13. In an electrodynamic transducer, a flat rectangular annulus, an elongated pole piece having a fiat elongated face on one side thereof, a bridge at a first of the fiat sides of said annulus joining the intermediate regions of two opposite parallel arms of said annulus, said bridge having two legs standing up one from each of said intermediate regions and a center portion connected to said legs at the free end of each, said pole piece being connected to the underside of the intermediate portion of said bridge, said bridge being dimensioned to provide that said flat elongated face lies in a plane which is substantially parallel to and at least in the close vicinity of one of the flat sides of said annulus, said fiat elongated face extending across said annulus substantially parallel to one pair of opposite parallel arms thereof, said pole piece .and said bridge being made of magnetizable material, substantially imperforate diaphragm means supported across the second flat side of said annulus hermetically sealing the area surrounded by said annulus, said diaphragm means having a mid-region confronting said flat elongated face of said pole piece, and a continuous ffat rectangular spiral of elongated electric conductor means attached to said diaphragm means and spreading over the major portion 'thereof substantially from said annulus to and including at least a portion of said midregion.

14. Electrodynarnic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed rectangular area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said flux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the

lines of force of said eld extending generally radially in direction between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and continuous elongated electric conductor means attached to said diaphragm means and extending over the major portion thereof substantially from said periphery to and including at least a portion of said mid-region, substantially all parts of said conductor means crossing said lines of force orthogonally and in the same sense relative to a given direction of current flow in said conductor means.

15. Electrodynamic Itransducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed rectangular 'area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said flux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force of said field extending generally radially in direction between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate diaphragm means disposed in and extending throughout said first-named area, and continuous elongated rectangular spiral electric conductor means attached to said diaphragm means and extending over the major portion thereof substantially from said periphery to and including -at least a portion of said mid-region, substantially all parts of said conductor means crossing said lines of force orthogonally and in the same sense relative to a given direction of current flow in said conductor means.

16. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed rectangular area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said fiux means comprising first pole means located substantially adjacent a mid-region of said area and second pole means located substantially adjacent at least a portion of the periphery of said area, the lines of force lof said field extending generally radially in direction between said first and second pole means in like sense from the periphery to said mid-region, the area of said first pole means confronting said first-named area being a minor fraction of said first-named area, substantially imperforate planar diaphragm means disposed in and extending throughout said first-named area, and continuous elongated flat rectangular spiral electric conductor means attached to said diaphragm means and extending over the major portion thereof substantially from said periphery to and including at least a portion of said midregion, substantially all parts of said conductor means crossing said lines of force orthogonally and in the same sense relative to a given direction of current flow in said conductor means.

17. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic fiux extending over a prescribed area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said means including first magnetic pole means having a flat pole face substantially parallel to said area and disposed immediately adjacent at least one of said limits confronting the mid region of said area, second pole means of annular form surrounding and defining said area, and Vmagnetizable nonpermanent magnet material located entirely outside said field joining said first and second pole means, said pole face having an area which is a minor fraction of said prescribed area, substantially imperforate diaphragm means mounted on said second pole means disposed in and extending throughout said area, and continuous elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof including the region thereof confronting said pole face, substantially all parts of said conductor means crossing the lines of force of said field orthogonally and in the same sense relative to a given direction of current flow in said conductor means.

18. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said means including first magnetic pole means having two spaced apart confronting flat pole faces of like polarity substantially parallel to each other and to said area disposed on opposite sides of said area one adjacent each of said limits confronting the mid region of said area, second pole means of annular form surrounding and defining said area, and magnetizable nonpermanent magnet material located entirely outside and on both sides of said field joining said first and second pole means, said pole faces each having a similar area which is a minor fraction of said prescribed area, substantially imperforate diaphragm means supported by said second pole means disposed in and extending throughout said prescribed area, and continues elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof including the region thereof between said pole faces, substantially all parts of said conductor means crossing the lines of force of said field orthogonally and in the same sense relative to a given direction of current flow in said conductor means.

19. Electrodynamic transducer according to claim 18 in which said second pole means comprises two similar annular members, and further comprising means to clamp ltJhe periphery of said diaphragm between said two memers.

20. Electrodynamic transducer according to claim 19 in which a separate permanent magnet provides each of said confronting pole faces and said material joining said first and second pole means comprises la separate support of said nonpermanent magnet material mounting each magnet on one of said members, the nonconfronting pole faces of said magnets being joined one to each of said supports.

21. Electrodynamic transducer comprising means to establish a substantially time-constant sheet-form field of magnetic flux extending over a prescribed elongated rectangular area and having substantially uniform density within prescribed limits measured normally from substantially any point in said area, said means including first magnetic pole means having two spaced apart permanent magnets of elongated rectangular form each having its poles on opposite long fiat side faces, said magnets being disposed on opposite sides of the mid region of said area with respective fiat faces bearing poles of like polarity confronting and parallel to each other and to said area, one of said respective flat faces being adjacent each of said limits, second pole means of magnetizable nonpermanent magnet material in rectangular annular form surrounding and defining said area, and magnetizable nonpermanent magnet material located entirely outside and on both sides of said field joining the nonconfronting poles of said permanent magnets with said second pole means, the areas of said confronting poles being alike and a minor fraction of said prescribed area, substantially imperforate diaphragm means supported by said second pole means disposed in and extending throughout said prescribed area, and continuous elongated electric conductor means attached to said diaphragm means and distributed over the major portion thereof including the region between said confronting pole faces, substantially all parts of said CTI conductor means crossing the lines of force of said eld orthogonally and in the same sense relative to a given direction of current iiow in said conductor means.

22. Electrodynamic transducer according to claim 2l in which said second pole means comprises two similar rectangular annular members, and further comprising means to clamp the periphery of said diaphragm between said two members.

23. Electrodynamic transducer according to claim 22 in which said material joining said nonconfronting poles with said second pole means comprises a first bridge spanning one of said rectangular annular members and connected at its ends between the midpoints of two opposite sides thereof and a second bridge spanning the other of said rectangular annular members and connected at its ends between the midpoints of two opposite sides thereof, and each of said nonconfronting poles is magnetically connected to the mid region of one of said bridges.

24. Electrodynamic transducer according to claim 23 in which the ends of each of said bridges are joined to the midpoints of the two short sides of the respective rectangular members spanned thereby and the mid region of each bridge is parallel to the magnet which is connected thereto.

25. Electrodynamic transducer according to claim 23 in which the ends of each of said bridges are joined to the midpoints of the two long sides of the respective rectangular members spanned thereby and the mid region of each bridge is substantially perpendicular to the magnet which is connected thereto.

26. For use in an electrodynamic transducer of the type having a sheet-form magnetic field, active diaphragm means comprising a substantially imperforate septum in the form of a readily iiexible sheet material coated with an elastomer, and continuous spiral-form elongated electric conductor means attached thereto and spreading over the major portion thereof from the periphery to and substantially including the mid region thereof.

27. Active diaphragm according to claim 26 in which said sheet material is a continuous sheet of a woven fabric.

28. For use in an electrodynamic transducer of the type having a sheet-form magnetic field, active diaphragm means comprising a substantially imperforate septum in the form of a supporting membrane, and continuous spiral-form elongated electric conductor means attached to one side of said membrane and spreading over the major portion thereof from the periphery to and substantially including the mid region thereof, and adhering to the other side of said membrane a layer of a relatively softer material substantially to dissipate vibrational energy due to exure of said membrane.

29. Active diaphragm according to claim 28 in which said relatively softer material is a woven fabric embedded in an elastomer.

30. Active diaphragm according to claim 28 in which said relatively softer material is a thin sheet of soft paper adhered to said membrane with a soft cement.

3l. Active diaphragm according to claim 28 in which mass loading means are attached to said membrane to adjust the natural resonance frequency thereof.

32. Active diaphragm according to claim 3l in which said mass loading means comprises an elongated mass member peripherally attached to said membrane.

33. Active diaphragm according to claim 32 in which elongated members exhibiting stiffness at frequencies above approximately 500 cycles per second are attached to said membrane spanning the same.

34. Active diaphragm according to claim 28 in which said supporting membrane comprises 'an electrically conductive sheet, and said conductors are affixed to said sheet and electrically insulated therefrom.

35. Active diaphragm according to claim 34 in which said conductive sheet is aluminum having a thickness in 1 7 the range approximately 0.2 to 2.0 mil. inches, and said conductors are aluminum Wire approximately 0.4 to 2.0 mil. inches in diameter.

36. Active diaphragm according to claim 28 in which said supporting membrane is a sheet of dielectric material approximately 0.2 to 2.0 mil. inches thick and said conductors are aluminum wire approximately 0.4 to 2.0 mil. inches in diameter.

37. Active diaphragm according to claim 28 in which said membrane is peripherally provided with motion compliance means made of sheet-form substantially irnperforate exible material, and said compliance means is peripherally provided with means to facilitate clamping the same to a solid body.

38. Active diaphragm according to claim 37 in which mass loading means are attached to said membrane at the periphery thereof to lower the natural resonance frequency thereof.

39. Active diaphragm according to claim 38 in which said mass loading means is an elongated mass member of low stiiness at audio frequencies in the range of said natural resonance frequency attached to said membrane at and extending continuously around the periphery thereof.

elongated members exhibiting stiffness at frequencies 40. Active diaphragm according to claim 39 in which 25 above approximately 500 cycles per second are attached to said membrane spanning the same.

References Cited in the file of this patent UNITED STATES PATENTS 1,238,439 Rummler Aug. 28, 1917 1,523,262 Gueritot Jan. 13, 1925 1,538,319 Gaumont May 19, 1925 1,676,668 Round July 10, 1928 1,704,511 Nakken Mar. 5, 1929 1,815,564 High July 21, 1931 1,955,390 Schiiil Apr. 17, 1934 2,404,798 Harry July 30, 1946 2,535,757 Root Dec. 26, 1950 2,565,591 Chichester Aug. 28, 1951 2,883,478 McConnell Apr. 21, 1959 3,013,905 Gamzon et a1. Dec. 19, 1961 FOREIGN PATENTS 1,169,701 France Sept. 15, 1958 1,049,917 Germany Feb. 5, 1959 1,218,259 France Dec. 14, 1959 OTHER REFERENCES Loudspeakers by G. A. Briggs, Third Ed., published by Wharfedale Wireless Works, Yorkshire, England, June 1950, page 12. 

1. ELECTRODYNAMIC TRANSDUCER COMPRISING MEANS TO ESTABLISH A SUBSTANTIALLY TIME-CONSTANT SHEET-FORM FIELD OF MAGNETIC FLUX EXTENDING OVER A PRESCRIBED AREA, SAID FLUX MEANS COMPRISING FIRST POLE MEANS LOCATED SUBSTANTIALLY ADJACENT A MID-REGION OF SAID AREA AND SECOND POLE MEANS LOCATED SUBSTANTIALLY ADJACENT AT LEAST A PORTION OF THE PERIPHERY OF SAID AREA, THE LINES OF FORCE OF SAID FIELD EXTENDING BETWEEN SAID FIRST AND SECOND POLE MEANS IN LIKE SENSE FROM THE PERIPHERY TO SAID MID-REGION, THE AREA OF SAID FIRST POLE MEANS CONFRONTING SAID FIRST-NAMED AREA BEING A MINOR FRACTION OF SAID FIRST-NAMED AREA, A SUBSTANTIALLY IMPERFORATE MEMBRANE COATED ON ONE SIDE 